With the development of data processing, a search has been made for materials exerting a great nonlinear optical effect for the purpose of realizing an optical theory element or optical switch on the basis of which photocomputers are developed. As nonlinear optical materials there have heretofore been known inorganic ferroelectric materials such as LiNbO.sub.3, BaTiO.sub.3 and KH.sub.2 PO.sub.4, quantum well structure semiconductors comprising GaAs, etc., organic single crystals such as 4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA) and 2-methyl-4-nitroaniline (MNA), conjugated organic high molecular compounds such as polydiacetylene and polyarylene vinylene, and semiconductor grain-dispersed glass comprising CdS, CdSSe, etc. dispersed in glass.
In particular, extensive studies have been made on semiconductor grain-dispersed glass as a favorable nonlinear optical material which exhibits both a high nonlinear optical susceptibility and a high response since Jain and Lind discovered in 1983 that a so-called color glass filter comprising semiconductor grains dispersed in glass exhibits a high three-dimensional nonlinear optical effect as described in J. Opt. Soc. Am., 73, 647 (1983).
The preparation of this kind of glass has been normally accomplished by a so-called melt-quenching method which comprises heat-melting a mixture of a powder of glass as a dispersant or starting material thereof and a powder of starting material of semiconductor or metal to make a glass melt, quenching the glass melt to around room temperature by casting on a metal plate or the like or like means to obtain a supercooled glass solid solution comprising semiconductor constituent elements dissolved therein as ions, and then subjecting the solid solution again to heat treatment at a proper temperature for a predetermined period of time to allow semiconductor grains to be separated out.
However, this method is disadvantageous in that it requires the starting material of semiconductor to be heated to a temperature as high as not lower than 1,000.degree. C. where the material can undergo decomposition or evaporation, limiting the kind of applicable semiconductors and the amount of semiconductor to be added. This prevents the realization of a material having a higher nonlinear optical effect for practical use.
As other methods there have been proposed a method which comprises the use of glass or SiO.sub.2 and an element semiconductor polycrystal such as CdS and CdT as a target in sputtering process to prepare a semiconductor grain-dispersed glass (as disclosed in J. Appl. Phys., 63 (3), 957 (1988), JP-A-2-307832), etc.
Further, an alternate method has been proposed which comprises the use of a high molecular compound as a matrix other than glass in a gas phase process such as vacuum metallizing to disperse finely divided grains of a semiconductor in the high molecular compound (as disclosed in JP-A-3-119326 and JP-A-3-140335). The gas phase process enables introduction of a large amount of semiconductor as compared to the above-mentioned melt-quenching method. In production of either inorganic matrix or organic matrix, the apparatus used for the gas phase process is expensive, and the speed of film formation is small so that it is suitable for formation of thin films but not thick films. Thus, the thus obtained element cannot be thick, resulting in limited applications.
As an approach for eliminating these difficulties there has been proposed a method which comprises dispersing and maintaining finely divided grains of a semiconductor or metal in a silica gel matrix formed by sol-gel method so that a semiconductor grain-dispersed glass can be prepared at a low temperature.
Examples of such a method include a method which comprises dispersing finely divided grains of a semiconductor prepared by CVD process or the like in a hydrolyzable solution of silicon alkoxide (sol), and then gelatinizing the sol so that the finely divided grains of a semiconductor are fixed in glass (as disclosed in JP-A-2-271933 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")), a method which comprises adding finely divided grains of a semiconductor to a sol containing a silane coupling agent or allowing the finely divided grains to be separated out in the sol, and then gelatinizing the sol so that the finely divided grains are fixated in glass (as disclosed in JP-A-3-199137), and a method which comprises forming a silica gel containing cadmium acetate, and then reacting cadmium acetate with hydrogen sulfate gas to allow cadmium sulfate grains to be separated out in the silica gel to obtain a semiconductor grain-dispersed glass as disclosed in the proceedings of The Ceramic Society of Japan's 1989 Annual Conference, lecture No. 2F20, J. Non-Cryst. Solids, 122, 101(1990)!.
However, tetralkoxysilane used in the conventional sol-gel process can easily crack at the stage of drying the gel. Tetralkoxysilane is also disadvantageous in that it cannot give a sufficient film thickness if it is formed into a thin film on a substrate to make an element. In order to obtain a film thickness enough for element, an approach has been employed which comprises repeating the steps of coating a substrate with a thin film to a thickness of not more than about 0.1 .mu.m, and then calcining the film at a temperature of hundreds of degrees C to give an appropriate film thickness.
Further, if as a method for dispersing finely divided grains of a semiconductor in a silica gel matrix formed by sol-gel process there is a method which comprises preparing finely divided grains of a semiconductor by separate methods, and then dispersing the finely divided grains in a sol, it disadvantageous in that the addition of the procedure for the preparation of the finely divided grains of a semiconductor complicates the process. It is also disadvantageous in that the finely divided grains used have a grain diameter as small as not more than hundreds of nanometer and thus can be hardly handled, giving an undesirable factor in the preparation process. These finely divided grains can easily agglomerate and thus can be hardly dispersed uniformly in the medium.
JP-A-2-271933 describes that the dispersion of finely divided grains can be effectively improved by ultrasonic dispersion or the addition of a surface active agent. However, ultrasonic dispersion inevitably involves the agglomeration of finely divided grains during the coating and drying in the formation of a thin film. The latter approach is disadvantageous in that the surface active agent thus added is decomposed or volatilized during the heat treatment, causing the re-agglomeration of finely divided grains.
In this respect, JP-A-3-199137 discloses the use of a silane coupling agent instead of surface active agent in an attempt to solve the problem of agglomeration of finely divided grains. In this approach, the silane coupling agent acts like a surface active agent and undergoes hydrolysis to connect to a matrix, making the material thermally stable and relatively undecomposable. JP-A-3-199137 proposes as a method for solving the problem of handling finely divided grains of a semiconductor to be added to a sol a method which comprises adding finely divided grains of a semiconductor in the form of solution to a sol, and then forming finely divided grains of a semiconductor in the sol with the aid of a solution of a paired ion source or reactive gas. However, this method is disadvantageous in that the finely divided grains thus separated out have a difficulty of diffusion, making it difficult to allow finely divided grains of a semiconductor having a quantum size effect to be uniformly separated out. Thus, the effect of this method leaves much to be desired.
Unlike the foregoing method, a method which comprises preparing a gel solid containing semiconductor material ions, and then subjecting the gel solid to post-treatment with hydrogen sulfate gas or the like to allow finely divided grains of a semiconductor to be separated out have no problems of complicated specification due to handling of finely divided grains or no difficulty of diffusion. However, since this method requires the use of a highly toxic gas such as hydrogen sulfate, it gives a very dangerous working atmosphere that requires a complicated process for safety. In the process which comprises post-treatment to allow finely divided grains to be separated out, as a starting material of finely divided grains of a semiconductor or metal there may be used a material soluble in a reactive medium which is then uniformly dissolved in a solution so that finely divided grains can be uniformly separated out in the medium. However, some materials have no proper solvent, limiting the concentration of finely divided grains which can be added.
Sol-gel method is a low-temperature process compared with melt-quenching, however, it requires to heat to a temperature of about 600.degree. C. Therefore, more low-temperature process is required to solve the problems such as decomposition of a semiconductor material by heating.
On the other hand, studies have been made on finely divided grains of a semiconductor or metal having a nonlinear optical effect. In particular, extensive studies have been made on the use of finely divided grains of cuprous halide which have excitons having a small Bohr diameter that can be effectively confined to give a great three-dimensional nonlinear optical effect (as described in Journal of Non-Crystalline Solids,. 134(1991), pp. 71-76, Journal of American Ceramic Society, 74(1991), pp. 238-240, Journal of Chemistry Society of Japan, No. 10 (1992), pp. 1231-1236). These cuprous halides are not soluble in a silane compound such as tetraethoxysilane Si(OCH.sub.2 CH.sub.3).sub.4 !, which has been heretofore used as a starting material of medium. Thus, the solvents in which these cuprous halides are soluble are limited. Further, since these cuprous halides have a low solubility, the amount of these cuprous halides which can be uniformly dissolved in a sol is low, making it impossible to allow finely divided grains to be separated out in a high concentration in a gel as a product. In general, the higher the concentration of finely divided grains to be added is, the higher can be expected the nonlinear optical effect. For the purpose of obtaining a material exerting a high nonlinear optical effect, a method has been sought for adding cuprous halides in a high density.
Further, if an easily oxidizable material such as cuprous halide is separated out, it undergoes deterioration such as oxidation and decomposition in a sol or during heat deposition, making it difficult to dope the sol with such a material.